Plasma etching is one of the most commonly employed techniques in the fabrication of semiconductor devices. It involves the selective removal of material by reactive free radicals or ions generated with a plasma. As it is well known in the art, plasma is an ionized gas in which concentrations of positive and negative ions are almost equal. The plasma may also contain free radicals which are electrically neutral yet highly reactive. Typically, a plasma is formed by introducing a predetermined gas into a plasma chamber and applying a radio frequency (RF) field to the plasma chamber. The gas introduced is chosen such that it will participate in the intended chemical reaction of a particular process. The RF field causes electron collisions with neutral or charged species to emit radiation. During the etching of semiconductor layer materials, halogen-containing compounds are commonly used in the gas phase as an etching gas to remove silicon based dielectric material.
One of the problems associated with the plasma etching process is the formation of particles which can cause defects due to particles fallen on the wafer surface. U.S. Pat. No. 5,362,356 discloses a passive, in-line method of monitoring film removal during plasma etching based on interference phenomena. In the method disclosed in the '356 patent, plasma emission intensity is monitored at a selected wavelength and variations in plasma emission intensity are correlated to the remaining film thickness, etch rate and uniformity, and etch selectivity. The method is most useful in conjunction with nitride island etch, polysilicon etch, oxide spacer etch, contact etch, etc; it can also be used in determining a particular remaining film thickness (e.g., just prior to clearing) at which point the etch recipe can be changed from a high-rate, low selectivity etch to a low-rate, high-selectivity etch.
U.S. Pat. No. 5,467,188 discloses a particle detecting system for detecting the number and size of particles generated in a process chamber of a semiconductor manufacturing system. The particle detecting system disclosed in the '188 patent includes a small detection chamber and a particle detector. The small detection chamber has an internal space thereof provided outside a wall portion forming the process vacuum chamber, and a plurality of laser beam transmitting windows and scattered light extracting windows. The particle detector is arranged in an atmospheric environment outside the detection chamber and including a laser diode for emitting a laser beam into the detection chamber through the laser beam transmitting windows and photosensors for detecting scattered light generated within the detection chamber through the scattered light extracting windows. One of the key element of the detection chamber of the '188 patent is that the structure is detachably attached to the process chamber. The particle detector is formed as a module, and can be attached to the detection chamber from outside the detection chamber. The particle detector has such a structure as to be detachably attached to the detection chamber.
U.S. Pat. No. 5,632,855 discloses a process for etching thermally grown oxide. The process involves a plurality of pre-stabilizing steps, followed by an etch step, which is then followed by a plurality of post-stabilizing steps. The post-stabilizing steps include a particle removal or byproduct flush step in addition to the post-stabilizing steps. The process parameters are chosen to remove thermal oxide within contact regions at a uniform rate. The resulting thermal oxide is substantially uniform with less than 3.0% variance in thickness across the contact regions and across like areas of the entire wafer surface. Bu utilizing the post-stabilize steps and the process parameters chosen for each step thereby provides an improved etch uniformity of thermal oxide films within fine-line areas.
U.S. Pat. No. 5,854,138 discloses a method for fabricating semiconductor and/or integrated circuit having reduced particulate count upon or within the circuit in which particles which formed within the plasma used to effectuate etch or deposition are gradually swept from the region above the integrated circuit, during power ramp down post etch or deposition. Plasma, and more specifically, the field which forms the plasma is maintained but at reduced levels to allow gradual reduction of particles through a multitude of steps. The steps culminate in eliminating power to the electrodes and plasma between the electrodes. However, at the time at which power is absent, only a few of the original particles remain in the critical region above the integrated circuit. Residual particles are removed in a purge step following the successive sequence of ramp down steps. Gap between the electrodes is increased to a final position early in the ramp down sequence so that additional electrode movement does not occur when the field is weakened.
The above prior art references illustrate the importance of controlling the particles formation during the semiconductor fabrication processes. However, no satisfactory solution has been offered to monitor the number density and size of the particles that may be formed during the plasma etching process. Light scattering technique has been well known in the art as a useful tool for measuring the number density and size of particles. However, light scattering technique has not been considered a viable approach for use in the plasma etching chamber, mainly because of the relatively low particle density and the high noise level, as a result of the constant changes in the component compositions and other dynamic nature in the plasma etching chamber, rendering the measured results highly unreliable.
The method disclosed in the '188 patent involves the addition of a small detecting chamber external to the main etching chamber. This may improve the sensitivity of the light scattering measurement, but it may also cause a disruption in the plasma flow pattern inside the etching chamber, and thus, may not be desired. U.S. Pat. Nos. 4,804,853, 4,739,177, 5,132,548 proposed the use of light scattering techniques in semiconductor fabrication processes. However, these techniques would suffer the same problems as discussed above if used in an etching chamber. Indeed, the '188 patent specifically mentioned that the techniques disclosed in these patents would adversely affect the quality of the semiconductor products.